The present invention relates generally to a method and a device for measuring the transmission loss in an optical waveguide, and particularly to a method and a device for measuring the transmission loss in an optical waveguide in an efficient way.
Optical waveguides for transmitting light wave are commonly used in integrated optical circuitries and optical communication systems, and the transmission loss in the optical waveguides are directly in relation to the performances of the circuitries and the communication systems. Accordingly, it is desirable to have a technique for efficiently measuring the transmission loss in an optical waveguide.
One conventional method, called the end fire guide-in/guide-out method, for measuring the transmission loss in an optical waveguide, includes coupling an incident light wave into an optical waveguide from a well-finished end face of the waveguide to let the light wave propagate through the waveguide, then coupling the light wave out of the waveguide from the opposite end face. By measuring the powers of the incident light wave and the outgoing light wave, we can obtain the transmission loss in the light wave. However, an optical waveguide is very thin, so that it is not easy to couple a light wave into the end face of the waveguide. Furthermore, the processing of cutting, polish, and lapping of the end face of the optical waveguide is time-consuming work. Besides, transmission loss in an optical waveguide can only be measured at its full length, and it is impossible to measure transmission losses at different locations of the same waveguide.
To overcome the disadvantages of the above-mentioned method, another method for measuring the transmission loss in an optical waveguide was suggested. As shown in FIG. 1, an incident light wave is coupled into a waveguide film 113 by way of a prism 115a which is coupled to the waveguide 113, and after propagating through certain distance, the light wave is coupled out of the waveguide 113 by way of another prism 115 coupled to the same waveguide 113.
However, the measurement result of the second method is dependent on the coupling coefficient between the prism and the waveguide, and the coupling coefficient is a function of the coupling force exerted on the waveguide by the prism. It is very difficult to keep all of the coupling forces at a constant value during measurements of the transmission loss in the films. Thus, the transmission losses in waveguide films of different lengths have not been precisely measured.
To eliminate the bad influences of the coupling coefficients, a third method is disclosed by Y. H. Won, P. C. Jaussand, and G. H. Chartier in a paper titled "Three-prism loss measurements of optical waveguides" of "Applied Physics Letters 37(3)" published on Aug. 1, 1980. As shown in FIG. 2, an incident light wave is coupled into the waveguide 13a by way of a prism 1, and the light wave propagating in the waveguide 13a is coupled respectively out of the waveguide 13a by way of a prism 2 and a prism 3. That is to say, a moveable prism 2 for guiding part of the incident light wave out of the waveguide 13a is disposed between the prism 1 and prism 3. Let P2 and P3 be the output powers of light waves coming out, respectively, from the prism 2 and the prism 3; r2 and r3 be the coupling coefficients, respectively, when the prisms 2 and 3 are coupled to the waveguide 13a; I(Z) be the intensity of the light wave propagating in the waveguide; Z be the location of the prism; .alpha. be the attenuation coefficient of the waveguide, then we will obtain the following equations EQU P2=r2*I(Z2) (1) EQU P3=r3*[I(Z2)-P2]exp[-.alpha.(Z3-Z2)] (2)
When r2=0, we will obtain P3=P30. When r2.noteq.0, we will obtain P3=P30-.DELTA.P3.
Using the above two equations (1), (2), we can eliminate r2 and r3 and obtain the following equation EQU I(Z)=(P2*P30)/.DELTA.P3=(P2*P30)/(P30-P3) (3)
It is possible to obtain the intensity of light wave propagating in the waveguide by using only the output powers P2, P3 of light waves coming out from the prism 2 and the prism 3, and thus the intensity of light wave propagating in the waveguide is independent of the coupling coefficients r2 and r3. If all the conditions surrounding the waveguide are kept constant, the transmission losses in the waveguide at different lengths can be measured by only moving the movable prism 2 and coupling it to the waveguide 13a at corresponding locations. However, three prisms should be used in the above method. It is necessary to space two prisms 2, 3 at a proper distance for guiding out the light wave out from the prism 2, thus the space left for the moving of the prism 2 is limited. Furthermore, the above method does not disclose any detailed mechanism for keeping the coupling coefficients constant.